1. Field of the Invention
This invention relates generally to the field of charge air boosting of internal combustion engines employing exhaust gas recirculation for emissions reduction and more particularly to a variable geometry turbine employing a four passage divided volute where two paired passages provide the necessary pressure differential to drive EGR, and the other two paired passages are relieved of the EGR-driving pressure differential requirement.
2. Description of the Related Art
Exhaust Gas Recirculation (EGR) has been the main technology used for NOx reduction in diesel engines in countries with stringent NOx emission targets. As Selective Catalytic Reduction (SRC) technology efficiency improves, it is supplanting EGR as the prime NOx reduction technology. However, SCR requires significant and wide spread infrastructure to support replenishment of urea for SCR engines as well as sophisticated sensor and control systems for On-Board Diagnostics (OBD) to prevent operators from tampering or circumventing the emissions control system.
Unfortunately, implementation of significant amounts of EGR result in a fuel consumption (and CO2 emission) penalty. The fuel consumption penalty is caused by the negative pressure gradient from the intake manifold to the exhaust manifold and the resulting pumping loss to the engine. As such, engine manufacturers have strived to innovate new boosting system technologies that are able to drive EGR while minimizing the fuel consumption penalty associated with the negative pressure gradient.
The main technology that has been used to create the negative pressure gradient while also maintaining the proper air/fuel ratio has been various types of Variable Geometry Turbocharger (VGT). A VGT provides a method of creating the appropriate negative pressure gradient while at the same time, increasing the boost so that additional gas flows through the engine. The additional gas is the diluent-cooled EGR. The EGR is additive to the fresh air that is needed for proper combustion, thus the total flow through the engine is increased. To increase the total flow through the engine, the density of the charge in the intake manifold must be increased, thus resulting in higher boost requirements from the boosting system.
As VGT is expensive and adds more failure modes to the engine system, some manufacturers have opted to use wastegated turbochargers, but with unequal volutes on the divided turbine housing; first volute 102 for receiving exhaust from engine cylinders not incorporating EGR and second volute 104 receiving exhaust from engine cylinders incorporating EGR as shown in FIG. 1. This is often referred to as “Asymmetrical Volute Turbocharging”, and is less expensive than VGT for driving EGR. It also provides some filet economy benefit as the engine's cylinders that are not required to provide EGR can operate at a much lower negative pressure gradient or even at a positive pressure gradient. However, the control for EGR and air fuel ratio is not as precise as the VGT system. The schematic for this type of system is shown in FIG. 2 wherein a diesel engine 202 (having six cylinders 204a-204f for the example) is provided with a turbocharger 206 having a compressor 208 and turbine 210 with the turbine driving the compressor through a shaft 212. A charge air cooler 214 receives charge air from the compressor section 208 and provides the charge air to the engine inlet manifold 216. Exhaust from the engine cylinders is provided from three non-EGR cylinders 204a-204c to a first exhaust manifold 218. The first exhaust manifold 218 is connected to the first volute 102. Exhaust from three EGR cylinders 204d-204f is provided to a second exhaust manifold 220. The second exhaust manifold provides EGR through an EGR cooler 222 and a control valve 224 into the inlet manifold 216. The second exhaust manifold also provides flow into the second volute 104. The first and second volutes 102 and 104 provide exhaust flow to drive the turbine 210 with a wastegate 226 providing boost pressure control. Prior art unequal volute systems require a wastegate for boost control and do not provide any secondary control capability for EGR tailoring.
It is therefore desirable to provide a variable geometry turbine for use with EGR which does not require a wastegate. It is additionally desirable to separately control EGR flow and boost.